Nice to see a bench without an apron - I am sure I will be told otherwise, but after 30 years of using a bench without aprons, I have yet to see the need for a 9"apron unless the projects you are working on weigh more than 2 metric tons.
It's perfectly stable - the top two baulks are 70mm African mahogany of some description, my late friend fished out of Hayle and got it out of the sea. It's like rock. The bottom rails are 4", but one is higher than the other - the shelf runs from the top of the front to a quadrant on the bottom of the one on the back. It helps stop "parallelogramming". I admit to being quite proud of it - I made it from a wheelchair. The handle on the end is to pull myself in if using a manual chair.
It sounds as if the "eye" on the operating screw is set too low. Take the bar with the eccentric cam out and give the "eye" it goes through a turn or two to raise it. A bit of trial and error and you'll get it right. Clean it and grease it before you put it back. You will be able to put put the bar back only when the cam is upwards - it tighten the tailstock as you turn it downwards.